Method and Apparatus for Detecting Frame Delimiters in Ethernet Passive Optical Networks with Forward Error Correction

ABSTRACT

Embodiments of the present invention provide a system that identifies an even delimiter in a forward error correction (FEC)-coded Ethernet frame. The system receives an FEC-coded Ethernet frame that includes the even delimiter, which is a predetermined sequence that separates a conventional Ethernet frame and FEC parity bits in the FEC-coded Ethernet frame. Next, the system scans a bit stream of the FEC-coded Ethernet frame. Then, the system determines a first Hamming distance between a first consecutive set of frame bits in the bit stream and the even delimiter. The system also determines a second Hamming distance between a second consecutive set of frame bits in the bit stream and the even delimiter. Both the first and second Hamming distances are shorter than a predefined value. The system subsequently selects one of the first and second sets of frame bits having the shorter Hamming distance as the even delimiter.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/223,359, filed Jul. 6, 2009, entitled “Method and Apparatus forDetecting Frame Delimiters in Ethernet Passive Optical Networks withForward Error Correction,” which is incorporated herein by reference inits entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This disclosure is generally related to the network design. Morespecifically, this disclosure is related to a method and apparatus fordetecting frame delimiters in Ethernet Passive Optical Network (EPON)frames with Forward Error Correction (FEC) code.

2. Background Art

EPONs have become a popular candidate for next-generation accessnetworks, because they offer the simplicity and scalability of Ethernetwith the cost-efficiency and high capacity of passive optics. Typically,EPONs are used in the first mile of the network that connects servicesubscribers to a central office of a service provider. However, sinceEPONs adopt passive optical transmission technology, which does notinvolve amplification or regeneration, the size of a network is subjectto power budget and various transmission impairments. As a result, thesignal-to-noise ratio of an EPON suffers as the network increases itssize, resulting in more frequent bit errors.

Forward error correction (FEC) techniques are often used to providerobustness against bit errors. With the FEC technique, a receivingdevice has the capability to detect and correct any block of symbolsthat contains fewer than a predetermined number of error symbols. Atransmitting device accomplishes FEC by adding bits to each transmittedsymbol block, using a predetermined error correction technique. Oneexemplary technique is the use of Reed-Solomon code. A Reed-Solomon codeis specified as RS(l, k) with s-bit symbols, which means that theencoder takes k data symbols of s bits each, and adds (l−k) paritysymbols to make an l-symbol codeword. A Reed-Solomon decoder can correctup to t symbols that contain errors in a codeword, where

2t=l−k. For example, RS(255, 239) with 8-bit symbols means that eachcodeword contains 255 bytes, of which 239 bytes are data and 16 bytesare parity. The decoder can automatically correct errors contained in upto 8 bytes anywhere in the codeword.

In order to ensure that FEC-coded Ethernet frames arebackward-compatible (i.e. recognizable by non-FEC-capable equipments),IEEE 802.3ah Ethernet in the First Mile standard proposes that the FECparity bits for all blocks of data symbols are aggregated and appendedto a conventional Ethernet frame. A delimiter that can be recognized bya non-FEC-capable equipment delineates the conventional Ethernet framefrom the parity bits. FIG. 1 illustrates an exemplary FEC-encodedEthernet frame format. In this example, an FEC-coded Ethernet framestarts with a start code sequence 210 (called “SFEC”). Following SFEC isan Ethernet frame, which includes a preamble/start-of-frame delimiter(SFD) field 120, a data frame 130, and a frame-check-sequence (FCS)field 170. FCS field 170 typically contains a cyclic redundancy check(CRC) sequence. Following FCS field 140 is a first delimiter TFEC 150indicating the end of the Ethernet frame. Another purpose of TFEC 150 isto delineate the Ethernet frame from the following FEC parity bits.

A byte of data is mapped to two 10-bit sequences (called “code groups”)in an EPON in order to maintain a balanced running disparity. Forexample, an octet of hexadecimal value 50 (01010000 in binary format) ismapped to code group 0110110101 and code group 1001000101. This pair ofcode groups is identified as “D16.2”. “D” indicates that this pair ofcode groups is used for data. “16” is the decimal value of the lowerfive bits of the octet (“10000”), and “2” is the decimal value of thehigher three bits of the octet (“010”). Besides data code groups, thereare also special code groups used for control purposes. For example,“K28.5” corresponds to code groups 0011111010 and 1100000101. Here, “K”indicates that it is a special code group, and “28.5” indicates thecorresponding octet value. In addition, the IEEE 802.3 standard alsodefines special control sequences (called “ordered sets”). For example,ordered set /I1/ (/K28.5/D5.6/) is the IDLE ordered set. It is definedsuch that the running disparity at the end of the transmitted /I1/ isopposite to that of the beginning running disparity. The IDLE orderedset /I2/ (/K28.5/D16.2) is defined such that the running disparity atthe end of the transmitted /I2/ is the same as the beginning runningdisparity. The first IDLE ordered set following a packet or a controlsequence restores the current positive or negative running disparity toa negative value. All subsequent IDLEs are /I2/ to ensure a negativerunning disparity. Other ordered sets include /R/ (Carrier Extend,/K23.7/), /S/ (Start of Packet, /K27.8/), and /T/ (End of Packet,/K29.7).

According to the IEEE 802.3 standard (for non-FEC-coded Ethernet), anend-of-packet delimiter should be either /T/R/ or /T/R/R/. The reasonfor having two delimiters is to ensure that the code group that followsthe delimiter falls in an even-numbered position. Therefore, /T/R/ isused when /T/ is in an even-numbered position, and /T/R/R/ is used when/T/ is in an odd-numbered position. Accordingly, as proposed in the IEEE802.3ah standard, TFEC 150 has two sequences: TFEC_E (/T/R/I/T/R) to beused when the first /T/ is in an odd-numbered position, and TFEC_O(/T/R/R/I/T/R) to be used when the first /T/ is in an even-numberedposition. Note that TFEC_E and TFEC_O include the conventionalend-of-packet delimiter (/T/R/ and /T/R/R/, respectively). Therefore, anon-FEC-capable receiving device can recognize the end of an Ethernetpacket. Following TFEC field 150 are the FEC parity bits 160. Accordingto the current 802.3ah standard, FEC parity bits 160 are based onRS(255, 239) codes derived from the Ethernet frame and do not protectTFEC field 150 against bit errors. After the FEC parity bits is anotherTFEC field 170 which terminates the entire FEC-coded frame. Because thebeginning of the parity bits is always in an even-numbered position dueto TFEC 150, and because the total number of parity bits is always even,TFEC 170 uses only the TFEC_E sequence.

FIG. 2 illustrates the code-group sequence of delimiters TFEC_E andTFEC_O according to the current IEEE 802.3ah standard. Frame 210 uses aneven TFEC delimiter, TFEC_E, as its delimiter between the conventionalEthernet frame and the FEC parity bits, because the first code group ofthe delimiter is in an even-numbered position. Frame 220 uses an oddTFEC delimiter, TFEC_O, as its delimiter between the conventionalEthernet frame and the FEC parity bits, because the first code group ofthe delimiter is in an odd-numbered position. In order to detect thedelimiter, a receiving device scans the input stream of symbols for amatch with TFEC_E or TFEC_O. Because the delimiter is not protected byFEC, a number of bit errors may be tolerated. According to the currentIEEE 802.3ah standard, up to five bit errors are tolerated in theprocess of matching TFEC delimiters. Furthermore, the current IEEE 802.3standard identifies the first sequence that has a shorter than 5-bitHamming distance to TFEC delimiters as the TFEC. Because the /T/R/pattern is repeated twice in the TFEC_E sequence, the TFEC_E sequencehas a high auto-correlation. As a result, it is prone to mismatch the/T/R/ pattern in the TFEC_E sequence under the current IEEE 802.3standard. The misidentification of TFEC_E will cause the receiver tomisrecognize the end of the frame.

BRIEF SUMMARY OF THE INVENTION

One embodiment provides a system that identifies an even delimiter in aforward error correction (FEC)-coded Ethernet frame. During operation,the system receives an FEC-coded Ethernet frame that includes an evendelimiter used to separate a conventional Ethernet frame and FEC paritybits in the FEC-coded Ethernet frame. The even delimiter is apredetermined bit sequence. The system then scans a bit stream of theFEC-coded Ethernet frame. Next, the system determines a first Hammingdistance between a first consecutive set of frame bits in the bit streamand the even delimiter. The system also determines a second Hammingdistance between a subsequent second consecutive set of frame bits inthe bit stream and the even delimiter. Both the first and second Hammingdistances are shorter than a predefined value. The system subsequentlyselects one of the first and second sets of frame bits having theshorter Hamming distance as the even delimiter.

In some embodiments, the even delimiter starts with first code groups/T/R/ according to the IEEE 802.3 Ethernet standard, and includes anumber of code groups after the first code groups /T/R/.

In some embodiments, the first and second consecutive sets of frame bitsare located within an enhanced delineator window, which specifies astart position and an end position in the bit stream.

In some embodiments, the enhanced delineator window is longer than adelineator window defined in the IEEE 802.3 Ethernet standard.

In some embodiments, the even delimiter ends with second code groups/T/R/ according to the IEEE 802.3 Ethernet standard.

In some embodiments, selecting one of the first and second sets of framebits having the shorter Hamming distance as the even delimiter reducesthe probability of misidentifying a first /T/R/ sequence in the bitstream, which corresponds to the first code groups /T/R/ in the evendelimiter, as a second /T/R/ sequence, which corresponds to the secondcode groups /T/R/ in the even delimiter.

In some embodiments, the even delimiter is used to separate theconventional Ethernet frame from the FEC parity bits when the lastsymbol of the conventional Ethernet frame is an odd-numbered position

In some embodiments, determining the second Hamming distance between thesubsequent consecutive set of frame bits in the bit stream is repeateduntil a best-matched set with the shortest Hamming distance to theeven-delimiter is found or the end of an enhanced delineator window isreached.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

FIG. 1 illustrates an exemplary FEC-encoded Ethernet frame format (priorart).

FIG. 2 illustrates the code-group sequence of delimiters TFEC_E andTFEC_O according to the current IEEE 802.3ah standard (prior art).

FIG. 3 illustrates an example of a potential misidentification of TFECsequence in accordance with the current IEEE 802.3 standard.

FIG. 4A illustrates an exemplary delineator window that a receivingdevice uses to detect a TFEC sequence in accordance with the prior arts.

FIG. 4B illustrates an improved delineator window that a receivingdevice uses to detect a TFEC sequence in accordance with an embodiment.

FIG. 5 illustrates how to calculate the length of FEC parity bits inaccordance with an embodiment.

FIG. 6 presents a flow chart illustrating the process of calculating thelength of FEC parity bits and subsequent data delineation in accordancewith an embodiment.

FIG. 7 presents a flow chart illustrating the process of identifyingTFEC_E using the best-match logic in accordance with an embodiment.

In the figures, like reference numerals refer to the same figureelements.

DETAILED DESCRIPTION OF THE INVENTION

The following description is presented to enable any person skilled inthe art to make and use the embodiments, and is provided in the contextof a particular application and its requirements. Various modificationsto the disclosed embodiments will be readily apparent to those skilledin the art, and the general principles defined herein may be applied toother embodiments and applications without departing from the spirit andscope of the present disclosure. Thus, the present invention is notlimited to the embodiments shown, but is to be accorded the widest scopeconsistent with the principles and features disclosed herein.

Overview

In embodiments of the present invention, the problem of potential TFEC_Emisidentification is solved by implementing an improved matching logicthat scans through an enhanced delineator window to identify the bestmatch to the TFEC_E sequence with the shortest Hamming distance.

As previously described, the current IEEE 802.3 standard identifies thefirst sequence in an incoming frame that has a shorter than 5-bitHamming distance to TFEC_E delimiters as the TFEC_E. Moreover, thestandard defines TFEC_E sequence as /T/R/I/T/R/. Because the /T/R/pattern is repeated twice in the TFEC_E sequence, the TFEC_E sequencehas a high auto-correlation. As a result, it is possible for the end ofa packet to match such criteria, i.e. with a Hamming distance to TFEC_Eless than a pre-defined value (e.g., 5 bits). FIG. 3 and the followingsection illustrate in details the problem with identifying TFEC underthe current IEEE 802.3 standard. Since the TFEC_E delineating the frameand the parity bits indicates the end of data frame, themisidentification of the TFEC_E sequence will cause the receiver tomisrecognize the end of the frame.

Embodiments of the present invention use an enhanced delineator window,which is longer than a conventional delineator window, to predeterminestart and end positions in an incoming bit stream to scan for a matchingTFEC_E sequence. The receiving device scans through all frame bitswithin the enhanced delineator window. While scanning the frame bits inthe bit stream, the receiving device temporarily stores potentialmatches for the TFEC_E sequence when the frame bits have a shorter than5-bit Hamming distance to TFEC_E. Then, the receiving device selects,among the potential matches, the frame bits within the enhanceddelineator window having the shortest Hamming distance to TFEC_E as thematching TFEC_E.

Problem with Identifying TFEC Using First-Match

FIG. 3 illustrates an example of a potential misidentification of TFECsequence using the match logic in accordance with the current IEEE 802.3standard. In this example, a transmitted bit stream 330 has a portionindicating the end of a frame 310 and a TFEC 320 following the end offrame 310. In this particular embodiment, the bits in the end of frame310 follow a data sequence of /D13.7/D23.1/D20.5/D29.5/. The TFEC 320 isan even TFEC, which has a special control sequence of/T/R/K28.5/D29.5/T/R/ under the IEEE 802.3 standard. Note that if arunning parity at the end of the conventional Ethernet frame isnegative, the number of code groups after the code groups /T/R/ includessequence /K28.5/D29.5/ according to the IEEE 802.3 Ethernet standard. Ifthe running parity at the end of the conventional Ethernet frame ispositive, the number of code groups after the code groups /T/R/ includessequence /K28.5/D10.1/ according to the IEEE 802.3 Ethernet standard.Although the data sequences at the end of the frame may vary from packetto packet, they would not significantly reduce the likelihood ofmisidentifying TFEC_E, because the problem is created by the intrinsiccharacteristic of high auto-correlation in the TFEC_E sequence.

Assuming that there was no transmission error in this example, theactual received stream 340 would be identical to the transmitted stream330. Next, assuming that there is a 1-bit match error 350 occurring ineach of the first three code groups respectively. That is, there existsa 1-bit match error between /D13.7/ (i.e., hex number 2C8) and /T/(i.e., hex number 2E8); a 1-bit match error between /D23.1/ (i.e., hexnumber 3A9) and /R/ (i.e. hex number 3A8); and a 1-bit match errorbetween /D20.5/ (i.e. hex number 0BA) and /K28.5/ (i.e. hex number 0FA).In other words, the Hamming distance between the bit sequence in theactual received stream 340, which starts from the end of frame 320, andTFEC_E would be less than 5 bits. Therefore, the match logic under thecurrent IEEE 802.3 standard would match the actual received stream 340to the matched stream 360 with 3 error bits in 60 bit correlation 380.As a result, the match logic under the current IEEE 802.3 standard wouldmisidentify the first 60 bits in the illustration as misidentified TFEC370 and cause a frame error.

Nevertheless, TFEC 320 in the transmitted stream 330 actually startsfrom the 5th code group. As shown in the illustration, there is no errorbit between the transmitted stream 330 and the matched stream 360 overthe 60-bit correlation 390 corresponding to the actual TFEC 320. Thebits corresponding to the 60-bit correlation 390 undoubtedly indicate abetter matched TFEC than the first-matched TFEC that corresponds to the60-bit correlation 380.

Using Best Match to Identify TFEC

FIG. 4A illustrates an exemplary delineator window that a receivingdevice uses to detect TFEC sequence (prior art). In this example, thereceiving device scans through a delineator window 460 to detect a SFEC110 at the start of a packet 410, a TFEC_E or TFEC_O 150 that delineatesbetween the end of data 420 and the start of parity 430, or a TFEC_Ethat follows the end of parity 450. Furthermore, the receiving deviceuses a matching logic that matches the first bit sequence with a Hammingdistance of less than 5 bits to SFEC, TFEC_E, or TFEC_O as theidentified FEC code.

FIG. 4B illustrates an improved delineator window that a receivingdevice uses to detect TFEC sequence in accordance with an embodiment ofthe present invention. In this embodiment, the receiving device uses thesame delineator window 460 to detect SFEC 110 before the start of packet410 and TFEC_E 170 following the end of parity 450. However, withrespect to the detection of TFEC_E 150 that delineates the end of data420 and the start of parity 430, the receiving device uses an enhanceddelineator window 470.

Enhanced delineator window 470 corresponds to a certain number of bitsin the incoming bit stream. The length of enhanced delineator window 470is greater than that of the conventional delineator window 460. Thisconfiguration allows the receiving device to scan for a longer stream ofincoming bits for the target TFEC_E. Therefore, it is more likely forthe receiving device to identify multiple potential matches for thetarget TFEC_E. Note that the receiving device identifies a potentialmatch for the target TFEC_E when the Hamming distance between aconsecutive set of the frame bits within enhanced delineator window 470and the TFEC_E is less than 5 bits.

The match logic under the current IEEE 802.3 standard identifies thefirst potential match within delineator window 460 as the target TFEC_E.As illustrated in FIG. 3, such match logic may result in matching thetarget TFEC_E prematurely, because it ignores other potential matchesthat can occur later within the delineator window. By contrast, thebest-match logic in accordance with the present invention implementsenhanced delineator window 470 that is longer than conventionaldelineator window 460. The best-match logic scans all frame bits betweenthe start and end positions as determined by enhanced delineator window470 for potential matches. Moreover, the best-match logic identifies thepotential match having the shortest Hamming distance to TFEC_E as thetarget TFEC_E. The identified target TFEC_E may not be the firstpotential match in the bit stream. Consequently, this enhanceddelineator window effectively reduces the likelihood of matching bits ofan incoming bit stream to TFEC_E prematurely by a receiving device.

FIG. 5 illustrates how to calculate the length of FEC parity bits inaccordance with an embodiment of the present invention. When thereceiving device scans the input stream, it is possible to learn thetotal length, X, of the Ethernet frame and the FEC parity bits, becauseSFEC 110 and the second TFEC delimiter 170 can be easily recognized. Thetotal length of the Ethernet frame is denoted as Y, and the length ofFEC parity bits 160 is denoted as Z. Because the FEC scheme uses RS(255,239) code, for every 239-byte block from the data symbol section (bitswithin Y), there is a 16-byte parity group in the parity section (bitswithin Z). Therefore, the length of the parity can be calculated as:

$\begin{matrix}{Z = {\left\lceil \frac{X}{\left( {m + n} \right)} \right\rceil \cdot n}} & (1)\end{matrix}$

where ┌ ┐ is the ceiling function;

m is the length of a block of bits from the conventional Ethernet frameand the first delimiter, which is 239; and

n is the length of a group of FEC parity bits corresponding to a blockof bits from the conventional Ethernet frame and the first delimiter,which is 16. Note that a ceiling function is used because Z may notcontain an integer number of 239 bytes, and because a block with lessthan 239 data bytes is padded up to have 239 bytes for the FECcalculation.

FIG. 6 presents a flow chart illustrating the process of calculating thelength of FEC parity bits and subsequent data delineation in accordancewith an embodiment of the present invention. The system within areceiving device starts by receiving an FEC-encoded Ethernet frame(i.e., the data stream after an SFEC up to the second TFEC) (operation610). Next, the system separates the Ethernet frame from thebest-matched TFEC (operation 615). The system further determines thetotal length of the conventional Ethernet frame, and first TFECdelimiter, and the FEC parity bits (operation 620). The system thencalculates the length of FEC parity bits based on equation (1)(operation 630). Based on the delineated FEC parity bits, the systemcorrects any possible bit errors that occur within the conventionalEthernet frame and the first TFEC delimiter (operation 640). Note thatthe first TFEC delimiter, if even, is identified using the best-matchlogic.

FIG. 7 presents a flow chart illustrating the process of identifyingTFEC_E using the best-match logic in accordance with an embodiment ofthe present invention. The system within a receiving device starts byreceiving an incoming FEC-encoded Ethernet frame in a bit stream(operation 710). Next, the system scans the bit stream of the receivedframe for a a potential match for TFEC_E (operation 720). In doing so,the system determines whether the hamming distance of a given number ofbits received so far (which is equivalent to the number of bits in aTFEC_E) and the TFEC_E is less than a threshold (operation 730). If so,the system stores the current bit position and the corresponding hammingdistance in a register (operation 740). Otherwise, the system continuesto receive the next incoming bit (operation 710).

The system then continues to receive and scan the next incoming bit(operation 750). After the next bit is received, the system determineswhether the hamming distance between the new bit group (which is shiftedby one bit due to the new incoming bit) and TFEC_E is less than thethreshold (operation 760). If so, the system further determines whetherthe new hamming distance is less than the one previously stored in theregister (operation 770). If the new hamming distance is shorter (whichresults in a “yes” value in operation 770), the system replaces the bitposition and hamming distance in the register with the current bitposition and hamming distance (operation 780).

If, however, the new hamming distance is greater than the threshold(which results in a “yes” value in operation 760) or greater than thehamming distance stored in the register (which results in a “no” valuein operation 770), the system keeps the previous hamming distance andbit position in the register, and determines whether the end of theenhanced delineator window has been reached (operation 790). If the endof the enhanced delineator window has not been reached, the systemcontinues to receive the next incoming bit (operation 750). If the endof the window has been reached, the system then identifies the bestTFEC_E position based on the bit position stored in the register,thereby providing the best-matched TFEC_E position (operation 792).

The data structures and code described in this detailed description aretypically stored on a computer-readable storage medium, which may be anydevice or medium that can store code and/or data for use by a computersystem. The computer-readable storage medium includes, but is notlimited to, volatile memory, non-volatile memory, magnetic and opticalstorage devices such as disk drives, magnetic tape, CDs (compact discs),DVDs (digital versatile discs or digital video discs), or other mediacapable of storing code and/or data now known or later developed.

The methods and processes described in the detailed description sectioncan be embodied as code and/or data, which can be stored in acomputer-readable storage medium as described above. When a computersystem reads and executes the code and/or data stored on thecomputer-readable storage medium, the computer system performs themethods and processes embodied as data structures and code and storedwithin the computer-readable storage medium.

Furthermore, methods and processes described herein can be included inhardware modules or apparatus. These modules or apparatus may include,but are not limited to, an application-specific integrated circuit(ASIC) chip, a field-programmable gate array (FPGA), a dedicated orshared processor that executes a particular software module or a pieceof code at a particular time, and/or other programmable-logic devicesnow known or later developed. When the hardware modules or apparatus areactivated, they perform the methods and processes included within them.

The foregoing descriptions of various embodiments have been presentedonly for purposes of illustration and description. They are not intendedto be exhaustive or to limit the present invention to the formsdisclosed. Accordingly, many modifications and variations will beapparent to practitioners skilled in the art. Additionally, the abovedisclosure is not intended to limit the present invention.

1. A method for identifying an even delimiter in a forward errorcorrection (FEC)-coded Ethernet frame, comprising: receiving anFEC-coded Ethernet frame that includes an even delimiter used toseparate a conventional Ethernet frame and FEC parity bits in theFEC-coded Ethernet frame, wherein the even delimiter is a predeterminedbit sequence; scanning a bit stream of the FEC-coded Ethernet frame;determining a first Hamming distance between a first consecutive set offrame bits in the bit stream and the even delimiter, wherein the firstHamming distance is shorter than a predefined value; determining asecond Hamming distance between a subsequent second consecutive set offrame bits in the bit stream and the even delimiter, wherein the secondHamming distance is shorter than the predefined value; selecting one ofthe first and second sets of frame bits having the shorter Hammingdistance as the even delimiter.
 2. The method of claim 1, wherein theeven delimiter starts with first code groups /T/R/ according to the IEEE802.3 Ethernet standard, and includes a number of code groups after thefirst code groups /T/R/.
 3. The method of claim 1, wherein the first andsecond consecutive sets of frame bits are located within an enhanceddelineator window, which specifies a start position and an end positionin the bit stream.
 4. The method of claim 2, wherein the enhanceddelineator window is longer than a delineator window defined in the IEEE802.3 Ethernet standard.
 5. The method of claim 1, wherein the evendelimiter ends with second code groups /T/R/ according to the IEEE 802.3Ethernet standard.
 6. The method of claim 4, wherein selecting one ofthe first and second sets of frame bits having the shorter Hammingdistance as the even delimiter reduces the probability of misidentifyinga first /T/R/ sequence in the bit stream, which corresponds to the firstcode groups /T/R/ in the even delimiter, as a second /T/R/ sequence,which corresponds to the second code groups /T/R/ in the even delimiter.7. The method of claim 1, wherein the even delimiter is used to separatethe Ethernet frame from the FEC parity bits when the last symbol of theEthernet frame is an odd-numbered position.
 8. The method of claim 1,wherein determining the second Hamming distance between the subsequentconsecutive set of frame bits is repeated until either a best matchedset with the shortest Hamming distance to the even-delimiter is found orthe end of an enhanced delineator window is reached.
 9. An apparatus foridentifying an even delimiter in a forward error correction (FEC)-codedEthernet frame, comprising: a processor; a receiving mechanismconfigured to receive an FEC-coded Ethernet frame that includes an evendelimiter used to separate a conventional Ethernet frame and FEC paritybits in the FEC-coded Ethernet frame, wherein the even delimiter is apredetermined bit sequence; a scanning mechanism configured to scan abit stream of the FEC-coded Ethernet frame; a determining mechanismconfigured to determine a first Hamming distance between a firstconsecutive set of frame bits in the bit stream and the even delimiter,wherein the determining mechanism is further configured to determine asecond Hamming distance between a subsequent second consecutive set offrame bits in the bit stream and the even delimiter, and wherein thefirst and second Hamming distances are shorter than a predefined value;a selecting mechanism configured to select one of the first and secondsets of frame bits having the shorter Hamming distance as the evendelimiter.
 10. The apparatus of claim 9, wherein the even delimiterstarts with first code groups /T/R/ according to the IEEE 802.3 Ethernetstandard, and includes a number of code groups after the first codegroups /T/R/.
 11. The apparatus of claim 9, wherein the first and secondconsecutive sets of frame bits are located within an enhanced delineatorwindow, which specifies a start position and an end position in the bitstream.
 12. The apparatus of claim 10, wherein the enhanced delineatorwindow is longer than a delineator window defined in the IEEE 802.3Ethernet standard.
 13. The apparatus of claim 9, wherein the evendelimiter ends with second code groups /T/R/ according to the IEEE 802.3Ethernet standard.
 14. The apparatus of claim 13, wherein the selectingmechanism reduces the probability of misidentifying a first /T/R/sequence in the bit stream, which corresponds to the first code groups/T/R/ in the even delimiter, as a second /T/R/ sequence, whichcorresponds to the second code groups /T/R/ in the even delimiter. 15.The apparatus of claim 9, wherein the even delimiter is used to separatethe conventional Ethernet frame from the FEC parity bits when the lastsymbol of the conventional Ethernet frame is an odd-numbered position.16. The apparatus of claim 9, wherein determining the second Hammingdistance between the subsequent consecutive set of frame bits in the bitstream is repeated until either a best matched set with the shortestHamming distance to the even-delimiter is found or the end of anenhanced delineator window is reached.
 17. A non-transitory computerreadable storage medium storing instructions which when executed by acomputer cause the computer to perform a method for identifying an evendelimiter in a forward error correction (FEC)-coded Ethernet frame, themethod comprising: receiving an FEC-coded Ethernet frame that includesan even delimiter used to separate a conventional Ethernet frame and FECparity bits in the FEC-coded Ethernet frame, wherein the even delimiteris a predetermined bit sequence; scanning a bit stream of the FEC-codedEthernet frame; determining a first Hamming distance between a firstconsecutive set of frame bits in the bit stream and the even delimiter,wherein the first Hamming distance is shorter than a predefined value;determining a second Hamming distance between a subsequent secondconsecutive set of frame bits in the bit stream and the even delimiter,wherein the second Hamming distance is shorter than the predefinedvalue; selecting one of the first and second sets of frame bits havingthe shorter Hamming distance as the even delimiter.
 18. Thenon-transitory computer readable storage medium of claim 16, wherein thefirst and second consecutive sets of frame bits are located within anenhanced delineator window, which specifies a start position and an endposition in the bit stream.
 19. The non-transitory computer readablestorage medium of claim 18, wherein the enhanced delineator window islonger than a delineator window defined in the IEEE 802.3 Ethernetstandard.